Nitrated aromatic hydrocarbons, such as nitrobenzene and nitrotoluene, are important chemical intermediates. Industrially, some nitration reactions are carried out with a molar excess of nitric acid over the aromatic compound, but others, for example the reaction of benzene and nitric acid to make nitrobenzene, operate with a molar excess of the aromatic compound. Examples in the patent literature of nitration processes for the production of nitroaromatic compounds where the nitration reaction is preferably carried out with a molar excess of the aromatic compound are Guenkel et al., U.S. Pat. No. 5,313,009 (e.g. production of mononitrobenzene); Konig et al., U.S. Pat. No. 5,648,565 (production of mononitrotoluene); and Demuth et al., U.S. Pat. No. 6,586,645 (production of nitrochlorobenzene).
Industrial applications of nitration reactions which operate with a molar excess of the aromatic compound would normally include a step for the recovery and recycling of the excess aromatic compound used (referred to herein as the ‘excess aromatic recovery’ step). This is the accepted industrial norm in the production of nitrobenzene, where a distillation column or live-steam stripper is used to recover the excess benzene from the produced nitrobenzene (typically, the excess aromatic compound is miscible in the produced nitrated product). The recovered excess benzene would normally contain some nitrobenzene.
Industrially, the aromatic compound to be nitrated will contain small quantities of non-aromatic compounds as impurities. In the case of benzene, examples of non-aromatic impurities include cyclohexane, methyl-cyclohexane, and ethyl-cyclopentane. The concentrations of these non-aromatic impurities will vary depending on the source of the aromatic compound. Many of these impurities do not nitrate and may degrade slowly in the nitration process, and, because of their organic nature, they mix with the produced nitrated product. Reference herein to ‘non-aromatic impurities’ means those non-aromatic impurities that do not nitrate in the nitration production train. A common physical attribute of these non-aromatic impurities is that they have boiling points close to that of the aromatic compound fed to the nitration reactor, or boiling points that lie somewhere between the boiling points of the nitrated product and the aromatic reactant. For example, benzene and cyclohexane have boiling points of 80° C. and 81° C. respectively, while the mono-nitrated product (mononitrobenzene) has a boiling point of 210° C. Table 1 lists non-aromatic impurities introduced by the benzene feed into a nitrobenzene plant known to the inventors and present in the recovered and recycled benzene stream of the same plant. Many of them have boiling points close to that of benzene or between the boiling points of benzene and mononitrobenzene.
TABLE 1NormalNormalBoiling PointBoiling PointCompound(° C.)Compound(° C.)2-methy-butane28cyclohexane81pentane362,3-dimethyl-pentane902,2-dimethyl-butane503-methyl-hexane92cyclopentane49heptane983-methyl-pentane63Methyl-cyclohexane101hexane69Ethyl-cyclopentane1042,2-dimethyl-792,4-dimethyl-hexane109pentane2,4-dimethyl-80Methylene-103pentanecyclohexane2,2,3-trimethyl-812,3-dimethyl-hexane116butane
As a result, a portion of the non-aromatic impurities is removed from the nitrated product in the excess aromatic recovery step (either distillation or live-steam stripping). Once removed from the nitrated product, these non-aromatic impurities mix with the recovered (excess) aromatic compound and are recycled back to the nitration reactor. Hence, the process naturally forms a closed loop where some non-aromatic impurities tend to build up. In general, the non-aromatic impurities building up in the process will be at their highest concentrations in the recovered and recycled aromatic stream. Once the recycled stream is introduced back into the reaction area, or back to storage, the non-aromatic impurities are diluted by the bulk of fresh aromatic compound addition.
In some cases, the build-up of these non-aromatic impurities can reach levels at which plant operation can be disrupted. Predicting whether build-up will disrupt production is very difficult. Some non-aromatics tend to degrade with time in the process. Once they degrade, their physical properties (e.g. vapor pressure, or acid solubility) change, giving the compounds a chance to naturally purge from the process. As a result, variables such as the types and concentrations of species of non-aromatic impurities and plant operating conditions play a role in whether build-ups will be sufficiently high to disrupt production or not.
The typical industrial method for removing non-aromatic impurities from a nitration plant to prevent build-up of the non-aromatic impurities from disrupting production is by purging. For example, the inventors are familiar with a nitrobenzene production facility where the build-up of non-aromatic impurities must be dealt with by having periodic purges of recycled benzene. Depending on the purge rate required, the loss of benzene and its disposal can be costly. Table 2 shows data for the concentration of non-aromatic impurities at different points in the process of that nitrobenzene production facility. These impurities, which amount to 330 ppm in the commercial benzene supplied to the plant, can build up by a factor of 45 in the feed to the nitration reactors and by a factor of over 500 in the recovered and recycled benzene.
TABLE 2Total Non-Aromatic ImpurityLocation of measurementConcentration (wt %)Benzene From Storage0.033Benzene To Reactor1.5(after benzene from storage and recycledbenzene are mixed)Recovered and Recycled Benzene17.7
Where a purge of the excess aromatic reactant is used to reduce the build-up of non-aromatic impurities, the purged stream would typically be sent for disposal. Mixing of the purged stream into the final nitrated product is typically not an option as this would affect the product quality, specifically in respect of the concentration of the residual non-nitrated aromatic compound. In the case of nitrobenzene production, this approach would lead to a product nitrobenzene with a benzene content that would exceed normally acceptable commercial specifications.
In a different industry, namely petroleum refining, there are processes for the removal of non-aromatics from aromatic streams by means of extractive distillation. Examples of these processes are described in Berg, U.S. Pat. No. 4,363,704 and U.S. Pat. No. 4,514,262. Extractive distillation differs from conventional distillation in that a solvent is added to increase the volatility ratio between the aromatic and non-aromatic compounds. In general, the further the relative volatility is from unity, the easier the separation is when using stripping or distillation. Removal of non-aromatics through extractive distillation is complex, usually involving more than one distillation tower to remove both heavy and light impurities, or a single tall distillation column with multiple draws. Typically, some of the solvent ends up with the aromatic compound, which depending on the solvent may become an operational nuisance for the end user. The extractive distillation process is therefore not an attractive option for removing non-aromatic impurities in a nitration process.
There is a need for a cost-effective method and apparatus for the removal of non-aromatic impurities from the recovered and recycled excess aromatic reactant from a nitration process.